This invention relates to an intrusion sensor and is particularly concerned with the kind of intrusion sensor in which a beam of radiation is established and an alarm given if the beam is at least partially interrupted. Sensors of this kind are often known as "fences" since they define a boundary which it is considered illegitimate to cross.
Fence-type intrusion sensors are known and are conveniently operated at microwave frequencies at which aerials for achieving reasonably well-defined beams become of practical size. In the systems so far proposed a transmitter and receiver are each set up with their respective aerials (which are assumed to be the same) aligned along the boundary at which the fence is to be erected. The most commonly required fence-type intrusion sensor is one in which the aerials are not more than a few feet above ground so as to establish a fence which would be penetrated by a person walking across the surveyed boundary and which is not so far off the ground that it could be crawled under. The fence should be high enough not to be stepped over but not so high that the movement of an intruder through the fence produces too small perturbation of the received signal for reliable detection. The microwave fence systems proposed to date to meet this requirement can give rise to an undue number of false alarms and it is found that there may be considerable difficulty in reliably setting up these fences at certain ranges. Non-reliable operation results in undue numbers of false alarms or a failure to give an alarm where a real intrusion occurs.
Investigations now made into these problems have led to the conclusion that a major reason for the difficulties encountered so far lies in the fact that, at least in the vertical plane, the aerials have relatively wide beam-widths and at the ranges required in practice act as point sources which, as will be explained later, causes difficulties due to ground reflection along the surveyed boundary. It will be shown that such systems are liable to be highly sensitive to ground reflection which may lead to a null being realised at certain ranges. In addition the ground-reflected component is highly sensitive to variations in the effective ground level or height. This in turn shifts the null ranges. In real situations microwave fences are often set up over irregular terrain and/or terrain which is in the open and has growing vegetation. At microwave frequencies vegetation such as grass affects reflection thus leading to seasonal variations in the effective ground level. Shorter term variations can arise out of vegetation moving in the wind.
As a result of the above investigations it has been concluded that a more predictable and reliable performance of a microwave fence could be achieved by making the system less sensitive to ground reflection.